The present invention relates to a novel class of cellulose derivatives which are mixed esters of a cellulose ether with acidic succinyl groups and aliphatic monoacyl groups, and to the enteric coating of pharmaceutical solid dosage forms of said cellulose derivatives and the prevention of halation on photographic films for which the cellulose derivatives are used.
Various known esters of cellulose per se or cellulose ethers with a carboxylic acid are useful for enteric coating and these include cellulose acetate phthalate, cellulose acetate succinate, methylcellulose phthalate, hydroxypropylmethylcellulose phthalate, methylcellulose succinate, hydroxypropylmethylcellulose succinate, and the like.
These cellulose derivatives can be used to form protective coatings on pharmaceutical solid dosage forms, particularly enterosoluble coatings, which do not dissolve in the gastric juice but readily dissolve in the intestinal juice.
However, the coating materials and enterosoluble coatings or enteric coatings produced therewith in the prior art have been found to be deficient in the following physiological inactivity to the human body, impossibility of producing toxic substances by decomposition, chemical or physical stability after the lapse of time, water resistance, i.e. stability against hydrolysis, or flexibility.
For example, cellulose acetate phthalate and cellulose acetate succinate in the powdered form or in the coating form are liable to decomposition by moisture, gradually liberating acetic acid, phthalic acid or succinic acid. Such decomposition taking place on the coated solid dosage forms results in a remarkable reduction of enterosolubility.
Further, methylcellulose phthalate and hydroxypropylmethylcellulose phthalate are defective in their flexibility as the coating material, and usually mixed with a large amount of a plasticizer. The use of plasticizers, on the other hand, brings about changes in the properties of the coatings formed due to migration of the plasticizer toward the surface of the coatings or gradual loss of the plasticizer by evaporation from the surface of the coatings with the lapse of time.
Furthermore, methylcellulose succinate and hydroxypropylcellulose succinate are free from the problem of insufficient flexibility of coatings and the disadvantageous use of plasticizers, but they are defective in that the coatings formed by these succinate derivatives adhere to each other. Additionally, the succinate derivatives tend to form lumps in the manufacturing process, especially, when the reaction product after completion of the esterification reaction is separated from the reaction mixture, thus bringing about difficulties in the purification of the reaction product.
Turning to halation prevention in photographic materials, photographic films and dry plates are invariably provided with a halation preventing layer containing colloidal carbon, graphite, dyestuffs and the like on the opposite surface of the layer of the photosensitive silver halide emulsion.
The coating compositions for providing the halation preventing layer contain a vehicle resin to act as a binder. Known vehicle resins are, by way of exemplification, copolymers of an ethylenically unsaturated carboxylic acid, such as acrylic acid-acrylic ester copolymers, and acrylonitrilemaleic anhydride copolymers; and cellulose derivatives, such as phthalates, tetrahydrophthalates, and hexahydrophthalates of a hydroxyalkylalkylcellulose, e.g. hydroxypropylmethylcellulose.
Among the above vehicle resins for the halation preventing layers, the copolymers of the unsaturated carboxylic acids are defective due to their limited solubility in organic solvents and in alkaline aqueous media, so that workability in the process of forming the halation preventing layer is relatively low and the removal or dissolution of the layer from the film substrate in the subsequent development process is effected only slowly.
Further, the above cellulose derivatives are not capable of producing coatings having sufficient flexibility and, accordingly, the photographic films provided with the halation preventing layers of the same cellulose derivatives are short of flexibility. Moreover, the cellulose derivatives themselves tend to be hydrolyzed in a humid atmosphere to form free acids, such as acetic acid, phthalic acid, tetrahydrophthalic acid, and hexahydrophthalic acid, resulting in degradating the halation preventing layers in the long run or adversely affecting the performance of photosensitive silver halide emulsion layers.